We can sit here and speculate all day about what caused them to split, but most fans assume that Kristen Stewart's cheating is probably what put an end to her relationship with Robert Pattinson.

According to Us Weekly, Rob just couldn't get over the thought of Kristen locking lips (and possibly more) with Rupert Sanders -- and he didn't hesitate to remind her of her actions whenever he got the chance.

One source said, "He really put her through the ringer. Ultimately, it was like, 'Either we move past this or we don't.' It wasn't good for either of them."

Another insider added, "Rob always brings up the cheating scandal to Kristen. He hangs it over her head."

Um, can you blame him? Hearing that your girlfriend cheatedon you with a married dude is never a good thing, but having the details of her infidelity plastered all through the media had to have added a new element of embarrassment into the mix that most regular couples don't have to contend with.

If what these sources say is true, then it sure sounds like Rob was harboring quite a bit of resentment for Kristen -- which is a clear sign that they never should've gotten back together in the first place. If you're willing to take someone back after they've cheated, the only way things will ever work going forward is if you wipe the slate clean and forgive them -- truthfully and wholeheartedly.

And that can be a pretty tough thing to do, especially considering the photos of Kristen Stewart making out with Rupert Sanders were everywhere after the cheating occurred. I'm sure it's been next to impossible for Rob to get those images out of his mind -- and based on their split, odds are good he'll never really get over what she did to him.

At this point, it's really best for both of them to move on with their lives and start dating other people. They've broken up not once, but twice now, presumably over Kristen's cheating, which makes it highly unlikely that they'd be able to survive another reconciliation for too terribly long. Bye bye, Robsten. We'll miss you. (Sort of.)